1. Field of the Invention
The present invention relates generally to rotary drill bits for drilling subterranean formations, and, more specifically, to rotary drag bits employing superabrasive cutting elements and employing longitudinally separated gage areas exhibiting varying aggressiveness with regard to side cutting a formation being drilled, so as to be easily steerable under combined axial and side loading while facilitating a smooth, ledge-free borehole wall in both linear and non-linear drilling.
2. State of the Art
It has long been known to design the path of a subterranean borehole to be other than linear in one or more segments, and so-called "directional" drilling has been practiced for many decades. Variations of directional drilling include drilling of a horizontal, or highly deviated, borehole from a primary, substantially vertical borehole, and drilling of a borehole so as to extend along the plane of a hydrocarbon-producing formation for an extended interval, rather than merely transversely penetrating its relatively small width or depth. Directional drilling, that is to say varying the path of a borehole from a first direction to a second, may be carried out along a relatively small radius of curvature as short as five to six meters, or over a radius of curvature of many hundreds of meters.
Perhaps the most sophisticated evolution of directional drilling is the practice of so-called navigational drilling, wherein a drill bit is literally steered to drill one or more linear and non-linear borehole segments as it progresses using the same bottomhole assembly and without tripping the drill string.
Positive displacement (Moineau) type motors as well as turbines have been employed in combination with deflection devices such as bent housing, bent subs, eccentric stabilizers, and combinations thereof to effect oriented, nonlinear drilling when the bit is rotated only by the motor drive shaft, and linear drilling when the bit is rotated by the superimposed rotation of the motor shaft and the drill string.
Other steerable bottomhole assemblies are known, including those wherein deflection or orientation of the drill string may be altered by selective lateral extension and retraction of one or more contact pads or members against the borehole wall. One such system is the AutoTrak.TM. system, developed by the INTEQ operating unit of Baker Hughes Incorporated, assignee of the present invention. The bottomhole assembly (BHA) of the AutoTrak.TM. system employs a non-rotating sleeve through which a rotating drive shaft extends to drive a rotary bit, the sleeve thus being decoupled from drill string rotation. The sleeve carries individually controllable, expandable, circumferentially spaced steering ribs on its exterior, the lateral forces exerted by the ribs on the sleeve being controlled by pistons operated by hydraulic fluid contained within a reservoir located within the sleeve. Closed loop electronics measure the relative position of the sleeve and substantially continuously adjust the position of each steering rib so as to provide a steady side force at the bit in a desired direction.
In any case, those skilled in the art have designed rotary bits, and specifically rotary drag, or fixed cutter bits, to facilitate and enhance "steerable" characteristics of bits, as opposed to conventional bit designs wherein departure from a straight, intended path, commonly termed "walk", is to be avoided. Examples of steerable bit designs are disclosed and claimed in U.S. Pat. No. 5,004,057 to Tibbitts, assigned to the assignee of the present invention.
It has been found that elongated gage pads exhibiting relatively low aggressiveness, or the tendency to engage and cut the formation, are beneficial for directional or steerable bits, since they tend to prevent sudden, large, lateral displacements of the bit, which displacements may result in so-called "ledging" of the borehole wall. A better quality borehole and borehole wall surface in terms of roundness, longitudinal continuity and smoothness is created, which allows for smoother transfer of weight from the surface of the earth through the drill string to the bit, as well as better tool face control, which is critical for monitoring and following a design path by the actual borehole as drilled.
This design approach exhibits shortcomings, however, if the available drilling system is only able to provide relatively low side loads, as is the case in otherwise highly sophisticated state-of-the-art steerable bottomhole assemblies relying upon integrally-powered active deflection elements rather than applied weight acting on the bit through a drill string including one of the aforementioned deflecting devices. "Relatively low" side loads include loads that are not sufficient to generate high enough contact stresses to fail the borehole wall material. In such a situation, the elongated gage pads limit the side cutting ability of the bit, and thus inhibit the ability of the bit to drill a non-linear path.
The conventional bit design approach responsive to limited side loads is to employ short or tapered gage pads to enhance the steerability of the bit. This approach, however, demonstrably lacks the directional stabilization and beneficial borehole condition-enhancing characteristics of the previously-described, elongated, non-aggressive gage pads.
Thus, there is a need in the directional drilling art for a steerable drill bit which provides good directional stability as well as steerability, precludes lateral bit displacement, and maintains borehole quality, all under relatively low side loads.